(1) Field of the Invention
The present invention relates to a speech coding transmission equipment. More particularly, it relates to a time domain harmonic scaling (TDHS) type high efficiency speech coding transmission equipment wherein a time domain harmonic scaling is applied by utilizing a repetition of a speech signal at each pitch period.
(2) Description of the Related Art
TDHS is a coding system wherein bandwidth compression and expansion in a time domain is carried out by utilizing a periodical characteristic of a speech waveform at each pitch period. The TDHS is described in detail in publications (1) and (2) shown hereinafter, is summarized in publication (3), and an improvement thereof is disclosed in publication (4).
A high efficiency speech coding transmission equipment uses the TDHS system to compress and expand the speech signal in the time domain, and thus maintain the quality of the signal.
In the transmission of speech signal by, for example, a mobile communication, satellite communication, intra-company communication, or the like, the cost of communication must be made as low as possible, and, for the storage of speech data in, for example, a speech signal storage unit, speech response system, or the like, the memory capacity must be made as small as possible. The above high efficiency speech coding transmission apparatus should satisfy these requirements.
In a high efficiency speech coding transmission equipment using the TDHS, however, a problem arises in that, when decoding speech signal, the clarity of the unvoiced speech therein is often degraded, and accordingly, there is a growing demand for a speech coding transmission equipment in which this degradation of the quality of the speech signal as a whole is avoided.
Reference Publications:
(1) D. Malah, "Time-domain algorithms for harmonic bandwidth reduction and time scaling of speech signals", IEEE Trans. Acoust. Speech Signal Processing, vol. ASSP-27, pp. 121-133 Apr. 1979.
(2) R. V. Cox et al., "An Implementation of Time Domain Harmonic Scaling with Application to Speech Coding", ICC 82, pp. 4G. 1. 1-4.
(3) S. Furui, "Digital Speech Processing", pp. 122-124, Tokai Daigaku Shuppankai (Japanese language publication).
(4) Morita, Itakura, "A Compression and Expansion System in Time Domain for Speech Signals using a Self-correlative Method, and an Evaluation thereof" Electric Acoustic Research Committee Material EA 86-5 (Japanese language publication).
In a conventional TDHS, evaluation functions for an extraction of the periodical characteristic of speech signals are calculated by a waveform correlation using the following equation (1) or by a waveform similarity using the following equation (2), and the period having the strongest correlation is determined to be a pitch period. EQU S.sub.1 (N)=.SIGMA.X.sub.j .multidot.X.sub.j-N /.SIGMA.X.sub.j.sup.2 ( 1) EQU S.sub.2 (N)=.SIGMA..vertline.X.sub.j -X.sub.j-N .vertline. (2)
The extent of the search for the pitch period is defined between an upper limit and a lower limit of the pitch frequency. For example, the search is carried out to the extent of 16.ltoreq.N.ltoreq.200.
However, an appropriate period cannot be extracted from unvoiced speech having no periodical characteristic by the above search methods. Therefore, since compression and expansion is carried out in accordance with a random pitch period, the characteristic of the waveform cannot be preserved, and therefore, the decoded speech signal is not clear, and thus a degradation of the quality of the transmission characteristics cannot be avoided.
The present invention intends to solve the above-mentioned problems in the conventional technology in this field.